The invention is directed to a method of controlling the pouring spout of a tundish for molten metals, and a casting apparatus for performing said method.
In continuous casting, the emphasis has been in controlling the pouring spout in such a way that a predetermined level of the bath surface in the mold and/or the tundish is achieved and maintained. For some time now, those skilled in the art have been concerned with the problem how the freezing of the pouring spout of a tundish, which is provided with a slide valve, can be prevented not only prior to tapping but also during casting interruptions or breakdowns of short duration. To this end the U.S. Pat. Specification No. 3,773,226 proposes to inject an inert gas into the pouring spout while the valve plate is in its closed state. Thereby the molten metal in the pouring spout is to be circulated in such a way that colder molten metal is mixed with warmer molten metal. In this way solidification of the molten metal in the pouring spout is prevented or at least delayed. But the disadvantages are, firstly, the relatively high operating costs due to the use of expensive inert gas, and secondly the fact that the molten metal is cooled by the injected cold gas. Due to the last-mentioned effect it may even happen in case of insufficient circulation of the molten metal that the latter freezes almost abruptly, even though the intention is to prevent such freezing. Finally, care must be taken that the gas is always injected at a pressure which overcomes the ferrostatic pressure in the pouring spout; otherwise there would be risk of the gas inlet orifices choking due to penetration of molten metal therethrough.
To avoid these disadvantages it is proposed in EP-A-66,118 that during casting interruptions the movable valve member of the slide valve in its closed position is oscillatingly driven in the direction of the opening and closing movement of the valve member. To this end the valve member is coupled to a vibrating means controlled by an oscillator. In many cases this last-mentioned proposal is sufficient; but in critical situations, i.e. in case of an unforeseeable prolonged casting interruption or breakdown, freezing of the molten metal cannot always be reliably excluded even by the use of said proposal.
One object therefore is the further improvement of the last-mentioned method and the associated casting apparatus so that also during critical periods of prolonged casting interruptions or breakdowns freezing of the molten metal in the pouring spout is reliably prevented.
It is furthermore desirable to prevent quiescent or no-flow zones in the pouring spout, especially in the vicinity of the slide valve, so as to avoid oxide deposits in said zones for example when pouring aluminium-killed steel. To this end it is proposed in the above-specified EP-A-66,118 that from time to time the valve member (valve plate) is moved from the choked valve position to the fully open position. Thereby the flow path of the molten metal through the valve is temporarily straightened. The oxide deposits, which in the choked state have formed preferably in the quiescent zones (shadow corners) are washed out. But a disadvantage of this process is the unavoidable variation of the free cross-section of the pouring spout, whereby a corresponding readjustment of the bath level in the mold becomes necessary. Moreover, the "washout" effect is relatively limited, especially when somewhat excessive time periods are selected between washing-out operations.
It is therefore a further object of the invention to control the pouring spout of a tundish for molten metal in such a way that deposits in quiescent zones will not be formed in the first place.
Finally, a still unsolved problem is the ingress of ambient air into the pouring spout between the abutting surfaces of the head plate the and bottom member of the tundish, on the one hand, and the valve plate and the head plate or the valve plate and the discharge member, on the other hand. The ambient air drawn into the pouring spout during casting is a serious metallurgical problem, because the proportion of additional nitrogen and oxygen is uncontrolled. When aluminium-containing steel is being poured, the ambient air drawn in promotes the formation and possible deposition of alumina, especially in the somewhat cooler region of the discharge member (immersion pipe). It is therefore another object of the present invention to resolve this problem or at least to alleviate it considerably.